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Nano structure formation by gas cluster ion beam irradiations at oblique incidence
Nuclear Instruments and Methods in Physics Research B 232 (2005) 212–216 www.elsevier.com/locate/nimb
Nano structure formation by gas cluster ion beam irradiations at oblique incidence Noriaki Toyoda *, Takahumi Mashita, Isao Yamada Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2, Kouto, Kamigori, Hyogo 678-1205, Japan Available online 28 April 2005
Abstract Nano structure formations on Au surfaces by Ar gas cluster ion beams (GCIB) irradiation at an oblique incidence were studied. When the incident angle was close to 0 from the surface normal of Au targets, the Au surface was smoothed due to the lateral sputtering effects and there were no structure formations. However, ripples were formed on Au surfaces at incident angle of 60 without sample rotation. By irradiations of GCIB at incident angle of 60 with sample rotation, cone like structures with 50 nm in diameters were fabricated. However, the surface roughness suddenly decreased above 60. Even though the surface roughness was the same, ripple structures were formed parallel to the incoming GCIB directions at 85 without sample rotation. The incident angle (h) dependence on the sputtering depth decreased following cosh dependence. Very efficient surface smoothing without removing materials were realized with oblique incidence of GCIB. 2005 Elsevier B.V. All rights reserved. PACS: 79.20.Rf; 81.65.Cf; 41.75.Ak Keywords: Cluster ion; Ripple formation; Oblique incidence
1. Introduction Recently, ion beam induced patterning or surface polishing attracts great interests for fabrication of nano-structures. The principle of ion
*
Corresponding author. Tel.: +81 791 58 0428; fax: +81 791 58 2666. E-mail address: [email protected] (N. Toyoda).
polishing or patterning is to move the atoms located near the top of a hill. Since atoms located on a hill have smaller binding energies than those in a valley, only the atoms on the hill can be moved without inducing dislocation of the atoms in the valley if an appropriate energy is supplied. Also the kinetic energy of the projectile ions should be sufficiently low so that atoms are not induced to move perpendicularly into the target. To achieve these aims, a glancing angle of
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.047
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216 Ar gas aperture ionizer
θ
nozzle
target
skimmer shutter
Fig. 1. Schematic diagram of GCIB system and experimental setup.
[12]. In this experiment, Ar was used as a source gas. After ionization of neutral Ar cluster beams, cluster ions were accelerated by an electrostatic potential of 20 kV. The cluster size distributions were measured with time of flight (TOF) technique. Fig. 2 shows cluster size distributions of Ar cluster ion beams at various nozzle pressures. Both average cluster size and intensity increased with Ar pressures in the nozzle. The average cluster size was about 1000 atoms/cluster at Ar pressure of 3800 Torr. In the GCIB system, a rotational stage was positioned on a beam axis and was inclined so that GCIB hit a sample at incident angle between 0 and 85 from the surface normal. The center of the rotational stage was aligned to the GCIB axis. Au films deposited by a vacuum evaporation were used as targets and surface morphologies after Cluster size [atoms/cluster] 0
1000
2000
3000
300
Ar cluster
200
3800Torr
Counts
incidence is frequently used for the surface smoothing with monomer ions. However, a high-fluence ion bombardment produces a variety of surface topological features. The most interesting features are ripple or wavelike structures developed on metals [1], semiconductors [2], Ge [3], GaAs [4] and insulators (SiO2 [5], glass [6]). These ripple formations are problematic in many applications, including secondary ion mass spectroscopy (SIMS) [7], depth profiling of Auger electron spectroscopy [8] and ion milling [9]. In a typical SIMS apparatus, the incident angle of the primary ions is oblique to obtain high sputtering yield and to avoid ion mixing. As sputtering proceeds, ripples are formed on the surface which induces degradation of the depth resolution. Ripple formation at glancing incidence of monomer ions was first modeled by Bradley and Harper [10]. They considered it as a competition between the local curvature dependence of the sputtering yield and the thermal diffusion of atoms on the surface. This theory agreed well with many experimental results using monomer ion beams. Recently gas cluster ion beam (GCIB) attracts interests for surface smoothing and near surface processing. Gas cluster ion is a huge aggregate with thousands of gaseous atoms and the sputtering phenomena with cluster ions are completely different from those with monomer ions, such as incident angle dependence of the sputtering yield and the angular distribution of sputtered atoms, termed Ôlateral sputteringÕ [11]. However, there is no data regarding ripple formation under gas cluster ion bombardment. Sputtering effects by cluster ions are strongly affected by the dense energy deposition in a local area, which creates crater and shockwave formation near the surface. In this study, incident angle dependence on surface morphologies by gas cluster ion beams was studied and nano-structure formations on Au surfaces were reported.
213
3000Torr 100
2300Torr 1500Torr 760Torr
2. Experiment 0
Fig. 1 shows an experimental setup for oblique irradiation of GCIB. Detail of GCIB system has been already reported in a previous review paper
0
40000
80000
120000
Mass [a.m.u] Fig. 2. Ar cluster size distribution at various nozzle pressure.
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216
Ar-GCIB irradiations were observed with an atomic force microscope (AFM). The acceleration energy of Ar-GCIB was fixed at 20 keV. These Au samples were irradiated with or without sample rotations. After irradiations, the sputtered depth was measured with a surface profilometer and the incident angle dependence of sputtered depth was obtained.
3. Results and discussions Fig. 3 shows an average roughness of Au surface measured with AFM plotted as a function of incident angles of Ar-GCIB. The surface roughness of as-deposited Au was 2.5 nm plotted as a dotted line. After irradiation of Ar-GCIB with ion dose of 1 · 1016 ion/cm2 at normal incidence, the surface roughness was decreased down to 0.9 nm by lateral sputtering effects. Even when the incident angle was increased to 30, the surface was still smooth with average roughness of 1.5 nm. However, when the incident angle was 60, the surface roughness dramatically increased to 9.4 nm. The AFM images of Au surfaces irradiated at 60 are shown in Fig. 4(a) (without sample rotations) and (b) (with sample rotations). Without rotation (Fig. 4a), ripples perpendicular to the incident directions of Ar-GCIB were formed, which caused drastic increase of the surface roughness as shown in the Fig. 3. The wavelength of rip-
14 Without rotation With rotation
12
Average roughness [nm]
214
10 8 6 4 Before irradiation (Ra=2.5nm)
2 0
0
20
40
60
80
100
Inicident angle [deg] Fig. 3. Incident angle dependence of average roughness of Au surface.
ples was about 150 nm. These ripples were also observed for Cu, Ag and Si by Ar-GCIB irradiations. When there was a sample rotation, cone like structures were formed on Au surface. The diameters of these nano-structures were about 50 nm. By changing the sample motion, formations of nano-structures such as grooves or cones can be controlled. Fig. 5(a) and (b) shows AFM images of Au surface after glancing angle (85) irradiations of Ar-GCIB. As same as the case at incident angle of 60, Fig. 5(a) and (b) correspond to that without
Fig. 4. Surface morphology of the Au surface (a) without and (b) with sample rotation after Ar-GCIB irradiation at incident angle of 60.
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216
215
Fig. 5. Surface morphology of the Au surface (a) without and (b) with sample rotation after 20 keV Ar-GCIB irradiation at incident angle of 85.
and with sample rotations, respectively. The average surface roughness was 1.25 nm (without rotation) and 1.15 nm (with rotation). As shown in the Fig. 5(a) (without rotation), there was a ripple formation, however, the direction of the ripples rotated 90 and became parallel to the incident direction of Ar-GCIB. These morphologies disappeared when the sample was rotated (Fig. 5(b)). It is expected that hillocks or swellings on a surface were preferentially removed by huge cluster ion impacts. Also the cluster ion enhanced lateral motions of atoms on the surface especially at glancing angles. When there was a sample rotation, the
motion of atoms became isotropic and the ripple like surface morphology disappeared. Fig. 6 shows an incident angle dependence of the sputtered depth of Au by Ar-GCIB irradiation. The sputtered depth was measured with a surface profilometer. The ion dose and the acceleration energy of Ar-GCIB were 1 · 1016 ions/cm2 and 20 keV, respectively. The sputtered depth was decreased following cos h from 0 with increasing the incident angles h. As the very smooth surface was realized at 85, surface smoothing without removing large amount of materials can be realized at glancing angle irradiation of GCIB.
100 Ar-GCIB 20keV Without rotation
Sputtering depth [nm]
80
cosθ
60
40
20
0
0
20
40
60
80
Incident angle [ deg] Fig. 6. Incident angle dependence of sputtering yield of Au by 20 keV Ar-GCIB.
4. Summary The incident angle dependence on Au surface morphologies by Ar-GCIB irradiations was studied. The surface was smoothed in the case of the normal incidence of Ar-GCIB, however, it drastically increased at 60 due to the ripple formations. When there was a sample rotation, the ripples formed without rotation became cone-like structure with 50 nm in diameter. In the case of 85, the surface roughness was significantly reduced. As the sputtering depth decreased with increasing the incident angle following cosh, very efficient surface smoothing was realized without removing large amount of materials. In the case of monomer ion beams, the ripple formations are strongly
216
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216
dependent on the acceleration energy, the energy dependence of Ar-GCIB on nano-structure formation must be studied in future. References [1] S. Rusponi, C. Boragno, U. Valbusa, Phys. Rev. Lett. 78 (1997) 2795. [2] G. Carter, V. Vishnyakov, Phys. Rev. B 54 (1996) 17647. [3] E. Chason, T.M. Mayer, B.K. Kellerman, D.T. McIlroy, A.J. Howard, Phys. Rev. Lett. 72 (1994) 3040. [4] E-H. Cirlin, J.J. Vajo, R.E. Doty, T.C. Hasenberg, J. Vac. Technol. A 9 (1991) 1395.
[5] T.M. Mayer, E. Chason, A.J. Haward, J. Appl. Phys. 76 (1994) 1633. [6] K. Oyoshi, S. Hishita, K. Wada, S. Suehara, T. Aizawa, Appl. Surf. Sci. 100 (1996) 374. [7] F.A. Stevie, P.M. Kahara, D.S. Simons, P. Chi, J. Vac.Technol. A 6 (1988) 76. [8] Eun-Hee Cirlin, Thin Solid Films 220 (1992) 197. [9] T.A. Winningham, Z. Zou, K. Douglas, N.A. Clark, J. Vac. Technol. A 16 (1998) 1178. [10] R. Bradley, J. Harper, J. Vac. Sci. Technol. A 6 (1988) 2390. [11] Z. Insepov, I. Yamada, Nucl. Instr. and Meth. B 99 (1995) 248. [12] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mat. Sci. Eng. R34 (2001) 231.
Nano structure formation by gas cluster ion beam irradiations at oblique incidence Noriaki Toyoda *, Takahumi Mashita, Isao Yamada Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2, Kouto, Kamigori, Hyogo 678-1205, Japan Available online 28 April 2005
Abstract Nano structure formations on Au surfaces by Ar gas cluster ion beams (GCIB) irradiation at an oblique incidence were studied. When the incident angle was close to 0 from the surface normal of Au targets, the Au surface was smoothed due to the lateral sputtering effects and there were no structure formations. However, ripples were formed on Au surfaces at incident angle of 60 without sample rotation. By irradiations of GCIB at incident angle of 60 with sample rotation, cone like structures with 50 nm in diameters were fabricated. However, the surface roughness suddenly decreased above 60. Even though the surface roughness was the same, ripple structures were formed parallel to the incoming GCIB directions at 85 without sample rotation. The incident angle (h) dependence on the sputtering depth decreased following cosh dependence. Very efficient surface smoothing without removing materials were realized with oblique incidence of GCIB. 2005 Elsevier B.V. All rights reserved. PACS: 79.20.Rf; 81.65.Cf; 41.75.Ak Keywords: Cluster ion; Ripple formation; Oblique incidence
1. Introduction Recently, ion beam induced patterning or surface polishing attracts great interests for fabrication of nano-structures. The principle of ion
*
Corresponding author. Tel.: +81 791 58 0428; fax: +81 791 58 2666. E-mail address: [email protected] (N. Toyoda).
polishing or patterning is to move the atoms located near the top of a hill. Since atoms located on a hill have smaller binding energies than those in a valley, only the atoms on the hill can be moved without inducing dislocation of the atoms in the valley if an appropriate energy is supplied. Also the kinetic energy of the projectile ions should be sufficiently low so that atoms are not induced to move perpendicularly into the target. To achieve these aims, a glancing angle of
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.047
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216 Ar gas aperture ionizer
θ
nozzle
target
skimmer shutter
Fig. 1. Schematic diagram of GCIB system and experimental setup.
[12]. In this experiment, Ar was used as a source gas. After ionization of neutral Ar cluster beams, cluster ions were accelerated by an electrostatic potential of 20 kV. The cluster size distributions were measured with time of flight (TOF) technique. Fig. 2 shows cluster size distributions of Ar cluster ion beams at various nozzle pressures. Both average cluster size and intensity increased with Ar pressures in the nozzle. The average cluster size was about 1000 atoms/cluster at Ar pressure of 3800 Torr. In the GCIB system, a rotational stage was positioned on a beam axis and was inclined so that GCIB hit a sample at incident angle between 0 and 85 from the surface normal. The center of the rotational stage was aligned to the GCIB axis. Au films deposited by a vacuum evaporation were used as targets and surface morphologies after Cluster size [atoms/cluster] 0
1000
2000
3000
300
Ar cluster
200
3800Torr
Counts
incidence is frequently used for the surface smoothing with monomer ions. However, a high-fluence ion bombardment produces a variety of surface topological features. The most interesting features are ripple or wavelike structures developed on metals [1], semiconductors [2], Ge [3], GaAs [4] and insulators (SiO2 [5], glass [6]). These ripple formations are problematic in many applications, including secondary ion mass spectroscopy (SIMS) [7], depth profiling of Auger electron spectroscopy [8] and ion milling [9]. In a typical SIMS apparatus, the incident angle of the primary ions is oblique to obtain high sputtering yield and to avoid ion mixing. As sputtering proceeds, ripples are formed on the surface which induces degradation of the depth resolution. Ripple formation at glancing incidence of monomer ions was first modeled by Bradley and Harper [10]. They considered it as a competition between the local curvature dependence of the sputtering yield and the thermal diffusion of atoms on the surface. This theory agreed well with many experimental results using monomer ion beams. Recently gas cluster ion beam (GCIB) attracts interests for surface smoothing and near surface processing. Gas cluster ion is a huge aggregate with thousands of gaseous atoms and the sputtering phenomena with cluster ions are completely different from those with monomer ions, such as incident angle dependence of the sputtering yield and the angular distribution of sputtered atoms, termed Ôlateral sputteringÕ [11]. However, there is no data regarding ripple formation under gas cluster ion bombardment. Sputtering effects by cluster ions are strongly affected by the dense energy deposition in a local area, which creates crater and shockwave formation near the surface. In this study, incident angle dependence on surface morphologies by gas cluster ion beams was studied and nano-structure formations on Au surfaces were reported.
213
3000Torr 100
2300Torr 1500Torr 760Torr
2. Experiment 0
Fig. 1 shows an experimental setup for oblique irradiation of GCIB. Detail of GCIB system has been already reported in a previous review paper
0
40000
80000
120000
Mass [a.m.u] Fig. 2. Ar cluster size distribution at various nozzle pressure.
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216
Ar-GCIB irradiations were observed with an atomic force microscope (AFM). The acceleration energy of Ar-GCIB was fixed at 20 keV. These Au samples were irradiated with or without sample rotations. After irradiations, the sputtered depth was measured with a surface profilometer and the incident angle dependence of sputtered depth was obtained.
3. Results and discussions Fig. 3 shows an average roughness of Au surface measured with AFM plotted as a function of incident angles of Ar-GCIB. The surface roughness of as-deposited Au was 2.5 nm plotted as a dotted line. After irradiation of Ar-GCIB with ion dose of 1 · 1016 ion/cm2 at normal incidence, the surface roughness was decreased down to 0.9 nm by lateral sputtering effects. Even when the incident angle was increased to 30, the surface was still smooth with average roughness of 1.5 nm. However, when the incident angle was 60, the surface roughness dramatically increased to 9.4 nm. The AFM images of Au surfaces irradiated at 60 are shown in Fig. 4(a) (without sample rotations) and (b) (with sample rotations). Without rotation (Fig. 4a), ripples perpendicular to the incident directions of Ar-GCIB were formed, which caused drastic increase of the surface roughness as shown in the Fig. 3. The wavelength of rip-
14 Without rotation With rotation
12
Average roughness [nm]
214
10 8 6 4 Before irradiation (Ra=2.5nm)
2 0
0
20
40
60
80
100
Inicident angle [deg] Fig. 3. Incident angle dependence of average roughness of Au surface.
ples was about 150 nm. These ripples were also observed for Cu, Ag and Si by Ar-GCIB irradiations. When there was a sample rotation, cone like structures were formed on Au surface. The diameters of these nano-structures were about 50 nm. By changing the sample motion, formations of nano-structures such as grooves or cones can be controlled. Fig. 5(a) and (b) shows AFM images of Au surface after glancing angle (85) irradiations of Ar-GCIB. As same as the case at incident angle of 60, Fig. 5(a) and (b) correspond to that without
Fig. 4. Surface morphology of the Au surface (a) without and (b) with sample rotation after Ar-GCIB irradiation at incident angle of 60.
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216
215
Fig. 5. Surface morphology of the Au surface (a) without and (b) with sample rotation after 20 keV Ar-GCIB irradiation at incident angle of 85.
and with sample rotations, respectively. The average surface roughness was 1.25 nm (without rotation) and 1.15 nm (with rotation). As shown in the Fig. 5(a) (without rotation), there was a ripple formation, however, the direction of the ripples rotated 90 and became parallel to the incident direction of Ar-GCIB. These morphologies disappeared when the sample was rotated (Fig. 5(b)). It is expected that hillocks or swellings on a surface were preferentially removed by huge cluster ion impacts. Also the cluster ion enhanced lateral motions of atoms on the surface especially at glancing angles. When there was a sample rotation, the
motion of atoms became isotropic and the ripple like surface morphology disappeared. Fig. 6 shows an incident angle dependence of the sputtered depth of Au by Ar-GCIB irradiation. The sputtered depth was measured with a surface profilometer. The ion dose and the acceleration energy of Ar-GCIB were 1 · 1016 ions/cm2 and 20 keV, respectively. The sputtered depth was decreased following cos h from 0 with increasing the incident angles h. As the very smooth surface was realized at 85, surface smoothing without removing large amount of materials can be realized at glancing angle irradiation of GCIB.
100 Ar-GCIB 20keV Without rotation
Sputtering depth [nm]
80
cosθ
60
40
20
0
0
20
40
60
80
Incident angle [ deg] Fig. 6. Incident angle dependence of sputtering yield of Au by 20 keV Ar-GCIB.
4. Summary The incident angle dependence on Au surface morphologies by Ar-GCIB irradiations was studied. The surface was smoothed in the case of the normal incidence of Ar-GCIB, however, it drastically increased at 60 due to the ripple formations. When there was a sample rotation, the ripples formed without rotation became cone-like structure with 50 nm in diameter. In the case of 85, the surface roughness was significantly reduced. As the sputtering depth decreased with increasing the incident angle following cosh, very efficient surface smoothing was realized without removing large amount of materials. In the case of monomer ion beams, the ripple formations are strongly
216
N. Toyoda et al. / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 212–216
dependent on the acceleration energy, the energy dependence of Ar-GCIB on nano-structure formation must be studied in future. References [1] S. Rusponi, C. Boragno, U. Valbusa, Phys. Rev. Lett. 78 (1997) 2795. [2] G. Carter, V. Vishnyakov, Phys. Rev. B 54 (1996) 17647. [3] E. Chason, T.M. Mayer, B.K. Kellerman, D.T. McIlroy, A.J. Howard, Phys. Rev. Lett. 72 (1994) 3040. [4] E-H. Cirlin, J.J. Vajo, R.E. Doty, T.C. Hasenberg, J. Vac. Technol. A 9 (1991) 1395.
[5] T.M. Mayer, E. Chason, A.J. Haward, J. Appl. Phys. 76 (1994) 1633. [6] K. Oyoshi, S. Hishita, K. Wada, S. Suehara, T. Aizawa, Appl. Surf. Sci. 100 (1996) 374. [7] F.A. Stevie, P.M. Kahara, D.S. Simons, P. Chi, J. Vac.Technol. A 6 (1988) 76. [8] Eun-Hee Cirlin, Thin Solid Films 220 (1992) 197. [9] T.A. Winningham, Z. Zou, K. Douglas, N.A. Clark, J. Vac. Technol. A 16 (1998) 1178. [10] R. Bradley, J. Harper, J. Vac. Sci. Technol. A 6 (1988) 2390. [11] Z. Insepov, I. Yamada, Nucl. Instr. and Meth. B 99 (1995) 248. [12] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mat. Sci. Eng. R34 (2001) 231.